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### R Draw Lines

`abline()` function adds a line to plot. It's expression is:

```abline(a = NULL, b = NULL, h = NULL, v = NULL, reg = NULL,
coef = NULL, untf = FALSE, ...)
```

`a,b`:Intercept and slope
`h`:for horizontal line
`v`:for vertical line
`...`

First let's make a plot:
```>x <- c(1.2,3.4,1.3,-2.1,5.6,2.3,3.2,2.4,2.1,1.8,1.7,2.2)
>y <- c(2.4,5.7,2.0,-3,13,5,6.2,4.8,4.2,3.5,3.7,5.2)
>plot(x,y,cex=.8,pch=1,xlab="x",ylab="y",col="black")
>x2 <- c(4.1,1.1,-2.3,-0.2,-1.2,2.3)
>y2 <- c(2.3,4.2,1.2,2.1,-2,4.3)
>points(x2,y2,cex=.8,pch=3,col="blue")
```

Let's add a red horizontal line at y=4 to the plot:
```>abline(h=4,col="red")
```

Let's add a green vertical line at x=0 to the plot:
```>abline(v=0,col="green")
```

Let's add a blue line with intercept 2 and slope 2 to the plot:
```>abline(a=2,b=2,col="blue")
```

`lty=` and `lwd=` control the line type and line width. There are 6 line types:

The line width can be a >0 number, for example, lwd from 1 - 8 as follows: